Field of the Invention
This invention is in the field of manufacturing fine lead titanate crystals by a chemical precipitation process, the crystals being useful as ferroelectric, piezoelectric or pyroelectric materials.
Description of the Prior Art
In the field of dielectric porcelain, new methods for the synthesis of fine dielectric oxide particles used as starting materials are being devised to meet the demand for small size electronic parts or other uses.
For example, in a multi-layer ceramic capacitor, it is necessary to reduce the thickness of the ceramic layer in order to increase the capacity while reducing the size and weight of the capacitor. It is accordingly necessary to comminute the starting dielectric oxides. From the standpoint of pressure resistance of the capacitors, extraordinary grain growth or non-uniform particle sizes at the sintering stage is to be avoided. Therefore, it is necessary to provide methods which are adapted for the synthesis of uniform sized particles.
In piezoelectric actuators, bimorph or pyroelectric LR sensors making use of piezoelectric or pyroelectric materials, there is an increasing demand for a uniform grain size. Above all, oriented ceramics would be preferred from the standpoint of manufacturing costs to oriented thin films obtained by high frequency sputtering, especially when the ceramics are to be used as sensor materials.
One of the most commonly used starting materials for dielectric oxides is lead titanate because of its numerous superior quality. Lead titanate, PbTiO.sub.3, is usually prepared by mixing lead oxide,PbO,with titanium oxide, TiO.sub.2, crushing the mixture in a ball mill, calcining the resulting product at 800.degree. to 1000.degree. C., again crushing to a uniform particle size, and sintering the resulting product.
In the preparation of fine powders of lead titanate in this manner, evaporation of PbO presents problems. With higher calcination temperatures, PbO evaporation is increased exponentially, thus changing the composition of the resulting fine lead titanate particles. In order to prevent this, some technique must be used during heat treatment, such as calcining the product in a PbO atmosphere. Although a lower calcination temperature may be used during sintering to reduce PbO evaporation, there may remain a considerable amount of the unreacted PbO at the time of terminating the sintering. Since the unreacted PbO is likely to be evaporated at the sintering stage, it becomes necessary to control the atmosphere. With fine lead titanate powders obtained by solid phase reaction relying upon heat treatment, A-site defects in the perovskite structure may be caused, resulting in less than stoichiometric amounts being present. Such non-stoichiometricity is very likely to affect piezoelectric or pyroelectric properties. Even in the event that high stoichiometricity is achieved by the high temperature heat treatment, the sintering may be affected by the above described process of preparing the starting materials. This is because the lead titanate exhibits the most pronounced crystal anisotropy among a variety of crystals having the perovskite structure, that is, that it shows an acute tetragonal strain with the result that it has a markedly different heat expansion coefficient in a given direction and hence is likely to crack during the lowering of the temperature. Various additives designed to improve the calcination properties may present problems in the piezoelectric properties. Lead titanate is effectively employed by making the best of its properties such that the electromechanical coupling factor of the longitudinal wave is larger than that of the transverse wave. There is, however, a risk that the addition of the impurities of the additive may lower the desired properties of the material. For this reason, lead titanate obtained by the solid phase reaction method is usually not used in its pure form but only for either its piezoelectric properties or calcination properties, which are contradictory to each other.
For a practical use of transparent ceramic starting materials, attempts have been made to improve the non-uniformity of the particle size, reduction in activity, and the manner of mixture of the impurities in order to obtain a uniform particle size.
For example, there is described in Japanese Patent Application No. 2080/1976 a wet synthesis method according to which the A-ion and B-ion salts of the perovskite structure ABO.sub.3 to be syntheiszed are reacted under boiling conditions in an aqueous alkaline solution. However, in this case, it is necessary that the ratio of A-ion to B-ion be equal to or greater than 1.8. In additiona, fluctuations may be caused because the synthetic ion concentration is not equal to unity. In addition, the post-synthesis product is amorphous and in the form of hydrates. Consequently, heat treatment at 300.degree. to 400.degree. C. is required to obtain crystalline particles, while excess PbCl.sub.2 which is an impurity at the time of synthesis must be removed by decantation, but this impurity is difficult to remove completely.
With the known oxalate method, it is difficult to obtain a product of uniform quality because of the different pH areas of precipitation or the different solubilities of the oxalates depending on the kinds of metal ions. The use of organic compounds such as oxalates presents problems in manufacturing costs and productivity.
In another known method, the oxalic acid ethanol method, while a certain improvement is achieved as to uniformity of the composition, there are problems concerned with manufacturing costs and productivity which are yet to be solved.
There is also known what is termed the metal alcoxide method according to which organometal compounds having the general formula M(OR)n are prepared and a composite alcoxide represented by the general formula M.sub.I M.sub.II (OR)m is prepared and hydrolyzed. The known method also presents problems in manufacturing costs and productivity. The resulting precipitates, while being pure, are amorphous and need to be heat-treated at about 400.degree. C.
To summarize, it is difficult in any of the known methods to synthesize crystalline fine particles or powders of lead titanate from the liquid phase without resorting to heat treatment, or to yield fine crystals of lead titanate with high purity and uniformity. It is completely impossible to control the shape of the fine lead titanate particles dependent upon the intended usage.